The present invention relates generally to solar collectors, and more particularly to a solar collector having a three-dimensional array of different types of monolithic photovoltaic cells enclosed within an enclosure having reflective surfaces to provide improved efficiency, extended operating life and reduced manufacturing cost.
Solar or photovoltaic cells (PVCs) are semiconductor devices having P-N junctions which directly convert radiant energy of sunlight into electrical energy. Conversion of sunlight into electrical energy involves three major processes: absorption of sunlight into the semiconductor material; generation and separation of positive and negative charges creating a voltage in the PVC; and collection and transfer of the electrical charges through terminal connected to the semiconductor material. PVCs are widely known and commonly used in a variety applications, including providing electrical energy for satellites and other space craft, marine vessels, installations in areas not served by a grid of an electric utility company, and portable consumer electronics devices such as radios, tape/compact disc players and calculators.
Heretofore PVCs have not been widely used as a main or even auxiliary source of power for residences and businesses having access to conventional power sources, for example, through a power grid of an electric utility company. There are several reasons for this, the most important of which is cost. Electricity produced from solar cells tends to be relatively expensive compared to that available from conventional power sources such as hydroelectric, oil-fired, coal fired and nuclear power plants.
Although the cost of installing, maintaining and repairing solar electric generation arrays or systems is not insignificant, the greatest cost associated with solar energy is the cost of the manufacturing the PVCs. Referring to FIG. 1, prior art PVCs 20 are typically formed on an ultra-pure silicon wafer or substrate 22, which in itself can cost from about 300 hundred to about 5 thousand dollars apiece depending on size. For example, an 8 inch diameter silicon commonly used in manufacturing PVCs typically costs about 2.5 thousand dollars. Furthermore, traditionally a large number of individual PVCs 20 were fabricated on a single substrate 22 by (i) depositing or growing a doped layer of semiconductor material, such as silicon, over the substrate 22 including a dopant of an opposite type; (ii) patterning and etching the substrate 22 with the doped layer thereon to form individual PVCs 20; (iii) depositing a metal layer over the etched substrate 22; (iv) patterning and etching the metal layer to form vias, contacts and lines interconnecting the individual PVCs 20; and (v) inspecting and testing the finished PVCs 20 to remove from an output circuit defective PVCs. The time, equipment and skilled operators required to perform each of the above steps makes the cost of solar electricity extremely expensive, and impractical for just about any use for which an alternative conventional energy source is available.
In an effort to reduce costs, some of the latest generations of PVCs have been monolithic PVCs in which substantially the entire surface of a substrate is taken up a by single large PVC, thereby eliminating much of the time and costs associated with patterning and etching the doped layer and the metal layer. However, this approach has not been wholly successful, since unlike with a substrate having numerous individual PVCs which can be individually removed from the output circuit, a single defect at any point in the monolithic PVC would render the entire substrate useless. In practice, this has resulted in yields well below 40%, offsetting or completely negating any cost savings realized with this approach.
Yet another problem with prior art PVCs is their efficiency in converting available light into electrical energy. This is particularly a problem for solar electric systems having limited power generating capability. That is, because usable solar energy is available for only a fraction of a day, when it is available the PVCs must generate energy to meet current demands and generate sufficient energy to be stored for use when usable solar energy is unavailable. Thus, conventional solar electric systems must either have relatively large numbers of PVCs, which as explained above are costly, or have a high degree of efficiency. Unfortunately, prior art PVCs are typically only from about 10 to 14% efficient.
Referring to FIG. 2 it is seen that a major reason for this poor efficiency is that a significant or even a large proportion of the light incident on a surface 24 of the PVC 20 is simply reflected away again. There have been several attempts in the prior art to remedy this including anti-reflective coatings on the surface of the PVC, and the use of a concentrator or lens 26 to enhance collection of incident light, as shown in FIG. 3. However, these solutions have not been wholly satisfactory for a number of reasons. One reason is that the addition of anti-reflective coatings or lens further increases the costs of fabricating the PVCs. More fundamentally, due to band-gap energy, which is a characteristic of every particular type of PVC, the PVC is capable of utilizing or converting into electricity only a narrow range of light wavelengths incident thereon, no matter how much light is concentrated on or prevented from being reflected from the surface of the PVC. For example, although solar radiation includes wavelengths from 2xc3x9710xe2x88x927 to 4xc3x9710xe2x88x926 meters, silicon based PVCs having a band gap energy of about 1.1 electron volts (eV) are capable of utilizing only wavelengths from about 0.3xc3x9710xe2x88x926 to about 3.0xc3x9710xe2x88x926 meters. Similarly, gallium-arsenide (GaAs) based PVCs, aluminum-gallium-arsenide (AlGaAs) based PVCs, and germanium (Ge) based PVCs have band gap energies of 1.43, 1.7 and 0.5 eV respectively, and are therefore sensitive to other wavelengths.
One possible approach to overcoming this inherit limitation of prior art PVCs 20 is shown in FIG. 4. FIG. 4 is a sectional side view of a PVC 20 having several separate layers 28, 30, 32, of semiconducting material to form several different p-n junctions, each having a different band gap energy and each sensitive to a different wavelength of light to enhance the utilization of incident light. Unfortunately, the additional processing steps required to fabricate this multilayer PVC makes the approach prohibitively expensive for all but those applications, such as satellites and spacecraft, for which no alternative exists. Moreover, certain types of PVCs, such as GaAs, AlGaAs or Ge based PVCs, are easily damaged by exposure to high levels of short wavelength or ultraviolet radiation.
Accordingly, there is a need for a solar collector that is inexpensive to fabricate, highly efficient in its utilization of available solar radiation, and which has an extended operational life.
The present invention provides a solution to these and other problems, and offers other advantages over the prior art.
It is an object of the present invention to provide a solar collector having an array of photovoltaic cells with improved efficiency, extended operating life and reduced manufacturing cost.
According to one aspect of the present invention, the solar collector includes a number of substrates arranged in a two-dimensional array of, each substrate having a monolithic photovoltaic cell (PVC) formed on a surface thereof for converting light incident thereon into electrical energy. The PVCs include at least two different types of PVCs receptive to different wavelengths of light and having different band gap energies. The array of substrates are enclosed within an enclosure having a top-wall with an anti-reflective coating through which light is passed to the PVCs, and bottom and sidewalls having reflective coatings to reflect at least a portion of light incident thereon onto the PVCs. Preferably, the enclosure further includes end-walls joining the top and bottom walls. Like the top-wall, the end-walls also have anti-reflective coatings thereon and join the top-wall at an angle selected to facilitate passage of light to the PVCs from a light source inclined relative to a surface of the top-wall.
In one embodiment, the PVCs include at least two different types of PVCs selected from a group consisting of silicon based PVCs, gallium-arsenide (GaAs) based PVCs, aluminum-gallium-arsenide (AlGaAs) based PVCs, and germanium (Ge) based PVCs. Preferably, where the PVCs include GaAs, AlGaAs or Ge based PVCs, the PVCs include a top passivation layer to filter damaging radiation.
In another embodiment, the solar collector further includes a voltage output circuit or circuit electrically coupling all the PVCs to a single voltage output from the solar collector. Generally, the circuit has a number of voltage converters to match voltages from the different types of PVCs to a common output voltage. The circuit can couple the PVCs in parallel, in series or in a combination of both. In one alternative embodiment, a number of a particular type of PVCs may be connected in series with one another and in parallel with a second number of a second type of PVCs having a different band gap energy to provide a common output voltage. For example, the solar collector can include 15 AlGaAs based PVCs having a band gap energy of 1.7 electron volts (eV), 18 GaAs based PVCs having a band gap energy of 1.4 eV, and 23 silicon based PVCs having a band gap energy of 1.1 eV to provide a common output voltage of about 25 volts direct current (vdc).
In another aspect the present invention is directed to a solar collector having a number of substrates arranged a three-dimensional array. Each substrate has at least one PVC formed on a surface thereof for converting light incident thereon into electrical energy. The three-dimensional array includes a lower or base-layer of substrates, and at least one elevated-tier of substrates positioned above and separated from the base-layer of substrates, so that at least a portion of the light passes between the substrates of the elevated-tier and is absorbed by the substrates of the base-layer.
In one embodiment, the elevated-tier includes substrates having surfaces with the PVCs formed thereon oriented to receive at least some of the light reflected from the substrates of the base-layer. Preferably, the PVCs are monolithic PVCs, and include at least two different types of monolithic PVCs selected from a group consisting of silicon, GaAs, AlGaAs, and Ge based PVCs. More preferably, the where the PVCs include GaAs, AlGaAs or Ge based PVCs, these PVCs are oriented to receive only light reflected from the substrates of the base-layer, thereby reducing their exposure to damaging levels of short wavelength or ultraviolet radiation. Optionally, the GaAs, AlGaAs and Ge based PVCs include a top passivation layer to filter-out or further reduce their exposure to damaging radiation.
In yet another aspect the present invention is directed to a solar collector including a three-dimensional array of substrates enclosed within an enclosure having a top-wall with an anti-reflective coating through which light is passed to the PVCS, and bottom and sidewalls having reflective coatings to reflect at least a portion of light incident thereon onto the PVCs. As above, each substrate has a PVC formed on a surface thereof, and the three-dimensional array includes a base-layer of substrates, and at least one elevated-tier of substrates positioned above and separated from the base-layer of substrates, so that at least a portion of the light passes between the substrates of the elevated-tier and is absorbed by the substrates of the base-layer.
In a preferred embodiment, the enclosure further includes end-walls joining the top and bottom walls, and the substrates of the elevated-tier are electrically coupled to and supported above the base-layer by a ground conductor affixed at both ends thereof to either the end-walls or the sidewalls of the enclosure. The ground conductor can include one or more wires or straps. Optionally, the substrates of the elevated-tier can be further supported by voltage conductors affixed the substrates and to the enclosure.
In one embodiment, the elevated-tier includes substrates having surfaces with the PVCs formed thereon oriented to receive at least a portion of light reflected from the substrates of the base-layer and/or from the bottom-wall of the enclosure. It will be understood that the solar collector can include multiple elevated-tiers, each having substrates on a top portion thereof and on a bottom portion thereof. The substrates on the top portion are oriented to receive light directly through the top-wall of the enclosure and light reflected from substrates on the bottom portion of an overlying tier. The substrates on the bottom portion of the elevated-tiers are oriented to receive light reflected from either substrates on the top portion of an underlying tier, the bottom layer of substrates, or the sidewalls and bottom-wall of the enclosure. Preferably, the elevated-tiers are offset from one another such that at some portion of the substrates of each elevated-tier and the bottom layer receive at least some light passed directly through the enclosure and onto the substrates.
In another embodiment, the PVCs include at least two different types of monolithic PVCs selected from a group consisting of silicon, GaAs, AlGaAs, and Ge based PVCs. Where the PVCs include GaAs, AlGaAs or Ge based PVCs, these PVCs are oriented to receive only light reflected from the substrates of the base-layer, thereby reducing their exposure to damaging levels of short wavelength or ultraviolet radiation. Optionally, the GaAs, AlGaAs and Ge based PVCs include a top passivation layer to filter-out or further reduce damaging radiation.
Generally, the solar collector further includes a circuit electrically coupling the PVCs to a voltage output from the solar collector, the circuit including a number of voltage converters to match voltages from the different types of PVCs to a common output voltage.
In yet another embodiment, the solar collector further includes a cooling mechanism selected from the group consisting of: (i) a number of vents in the enclosure to enable movement of air therethrough; (ii) vents in the enclosure and a fan to facilitate movement of air through the enclosure, the fan powered by at least a part of the voltage output from the PVCs; and (iii) a heat exchanger thermally coupled to at least some of the substrates and/or the enclosure, the heat exchanger including one or more passages or tubes through which a fluid is passed to cool the solar collector. In one preferred version of this embodiment, the heat exchanger is adapted to provide heat or heated water, in particular potable water, to a residence or business.
Advantages of the solar collector of the present invention include any one or all of the following:
(i) an improved efficiency of up to 3 times that of similarly sized conventional solar collectors;
(ii) reduced size or xe2x80x98footprintxe2x80x99 as compared to conventional solar collectors with a similar power output, thereby simplifying an installation process and enabling use of the inventive solar collector in locations having a limited area available for a solar cell;
(iii) extended operating life made possible by reducing exposure of sensitive PVCs to damaging levels of short wavelength or ultraviolet radiation, and by actively cooling the solar collector to maintain the PVCs below a maximum desirable operating temperature;
(iv) ability to use fluid from a heat exchanger used to cool the solar collector to provide heat or heated water to a residence or business;
(v) reduced manufacturing or fabrication cost made possible by use of monolithic PVCs thereby eliminating the need to form and interconnect multiple PVCs on a single substrate; and
(vi) reduced manufacturing time achieved by eliminating the need to form and interconnect multiple PVCs on a single substrate.